Atmospheric turbulence, or refractive-index fluctuations, along the path of a partially coherent beam can result in intensity fluctuations at the receiving end of the propagation path. At the pupil plane this phenomena can create a characteristic beam breakup, which can result in the appearance of “blobs” or “speckles” at the pupil plane, and which is referred to in this paper as “speckles”, (as used herein, the term “speckles” is not to be confused with the speckle pattern created due to the scattering of a beam from a rough surface). These intensity fluctuations can severely limit the performance of free-space optical communication; thus, their characterization is of importance. One method of measuring atmospheric turbulence is by estimating Fried's coherence length, r0, through its relationship with the number and size of discrete speckles captured on an image plane after propagation through a turbulent field.
In the medical fields, “blob” identification is becoming an important means of automating image analysis. These “blobs” can be representative of cells, bacteria, etc., in various types of images, and counting methods are able to count cells, detect blood vessel structures, map brain activity, and complete other manually tedious tasks. Considering the large effort put into optimizing these counting methods, these interdisciplinary counting techniques can be leveraged to quickly and effectively count speckles in an image from the receiver of a laser communications systems. The speckles can be due to atmospheric turbulence, and if the speckles can be counted, the atmospheric conditions can be characterized, which can allow for additional information such as maximum effective range of the communications system to be characterized.
In view of the above, it can be an object of the present invention to provide a method for characterizing atmospheric turbulence along an optical path without using a scintillometer. Another object of the present invention can be to provide a method for characterizing atmospheric turbulence along an optical path by adapting cell counting methods from the life sciences to count speckles along the optical path. Still another object of the present invention can be to provide a method for characterizing atmospheric turbulence along an optical path, in order to gage the coherency of a laser beam in the turbulence. Yet another object of the present invention to provide a method for characterizing atmospheric turbulence along an optical path that can be easy to accomplish and use in a cost-effective manner.